We propose the creation of 10,000 knock-out alleles of mouse genes exclusively by gene targeting. The generation of mutant mice from C57BL/6-derived embryonic stem cells is currently not sufficiently robust for this project. Therefore we will start with ES cells derived from a highly-efficient 129 mouse strain, switching to C57BL/6 ES cells as soon as they have been validated for high-throughput gene targeting. The work will be done by a three member consortium of the Children's Hospital Oakland Research Institute in Oakland, CA (CHORI), the Wellcome Trust Sanger Institute, Hinxton, Cambridge, UK and the University of California Davis Mouse Biology Program (UCD-MBP). Targeting vectors will be created at CHORI by recombineering of BAG clones and will contain exchangeable modules based on Gateway[unreadable] technology. The vectors will be transferred into ES cells at Sanger and sequenced in full. Each target locus will be tagged with a lacZ reporter cassette that effectively disrupts the gene to create a null allele. Over 1 million ES cell colonies will be robotically arrayed during the course of the project. Allele structures will be confirmed through long-range PCR for up to 100 ES cell clones per targeting experiment. Five independent knock-out mutants will be archived for each gene to maximize the likelihood of germ-line transmission. Comprehensive quality assurance testing of 300 ES mutant lines annually in vitro and in vivo will take place under the direction of the MBP-UCD to ensure pluripotency, establish germ-line transmission, and confirm viability of live mice and cryopreserved embryos and sperm. Mutant ES cells and embryos will be split and placed in cryo-storage between Sanger and UCD, while modular targeting vectors will be stored at CHORI.